How to find the MainWindowHandle of a WinForms process for capturing a snapshot of the whole window instead of capturing only the popup window

Question

Unable to capture the entire WinForms window when the popup/Dropdown window is open. It only captures a snapshot of the popup window. The WinForm process mainwindow handle retrieves only the dropdown menu handle instead of the handle that captures the whole window.

var process = Process.GetProcessesByName("136").FirstOrDefault();
            {
                IntPtr handle = process.MainWindowHandle;
                
                handles.Add(handle);

            }

I have identified a list of handles in the WinForms process by using the process name. How can I identify which handle is the main window handle, without using the window caption (title), that covers the entire WinForms window for capturing the snapshot? This is because having one or more windows with the same window title can lead to conflict.

I have tried to resolve the issue of capturing only the popup window by using the User32 'IsWindow' method, but it did not work. It retrieves more handles for capturing the snapshot. How do you identify which handle captures the entire window and which one is the MainWindow handle for the WinForms Window.

private IntPtr[] GetAllWinFormsWindowHandles()
        {
            List<IntPtr> handles = new List<IntPtr>();
            
            foreach (var hwnd in GetWindowHandlers("136"))
            {
                string windowTitle = GetWindowTitle(hwnd);
                RECT windowRect;
                if (GetWindowRect(hwnd, out windowRect))
                {
                    if (windowRect.left != windowRect.right && windowRect.top != windowRect.bottom)
                    {
                        if (User32.IsWindow(hwnd) )
                        {
                            handles.Add(hwnd);

                        }
                    }
                }
            }
            return handles.ToArray();
        }

private IEnumerable<IntPtr> GetWindowHandlers(string processName)
        {
            Process[] p = Process.GetProcessesByName(processName);
            var handle = p[0].MainWindowHandle;
            List<IntPtr> handles = new List<IntPtr>();
            foreach (ProcessThread thread in p[0].Threads)
            {
                User32.EnumThreadWindows(thread.Id, (hWnd, lParam) =>
                {
                    handles.Add(hWnd);
                    return true;
                }, IntPtr.Zero);
            }
            return handles;
        }

Capturing the Popup window

Expected image while capturing the snapshot